Males of the poecilliid fish, Xiphophorus pygmaeus, vary in size from 21 - 42 mm of standard length. Work is in progress to determine whether this variation is controlled by environmental or genetic factors. To this date our evidence indicates that a sex-linked gene locus controls growth rate and age of sexual maturity. At maturity males stop growing. This species is polymorphic at his locus. Two alleles have been identified. Males with the allele for slow growth rates reach a maximum size of 24 mm and mature under standard conditions at 17 weeks; those with the allele for fast growth rate average from 33 - 42 mm and reach maturity around 29 weeks. Investigations are in progress to determine whether the site of gene action is at the level of the pituitary gland.